How do I make a mirror?
For Blender Cycles 

Go to the materials tab, create a new material, where it says diffuse click and press glossy BSDF. Set the roughness to ~0.



 For Blender Internal 

Go to the materials tab, create a new material, tick the mirror box and set reflectivity to 1 or a high number.



This is for the most basic mirror which just reflects. To make a more complicated one, learn a bit more about materials for Blender Internal and for Cycles learn about the types of shaders and the mix node.